A synchronous machine is a dynamoelectric machine which may be utilised as a motor for driving a shaft or any load at a constant speed or as a generator for producing a voltage at a predetermined frequency depending on the speed of the driving shaft. When the device is used as a synchronous generator, it is customary to e.g. provide field excitation for the rotor through a synchronous brushless exciter generator. The exciter generator converts the direct current (DC) stator field into a polyphase alternating current (AC) armature voltage which is rectified by a set of rotating rectifiers mounted on or within the driving shaft to provide the DC excitation for the field windings of the synchronous generator, i.e. for the rotor of the generator.
Thus, a rotating exciter is a reversed generator with the field winding, fed with DC current, implemented on the static parts. The armature is located on the rotating part and produces AC voltage. A set of diodes is used to rectify it to produce a DC current, required to energise the field winding of the synchronous machine, i.e. the rotor of the generator.
As an order of magnitude, the power generated by the exciter is 0.5 to 2% of the rated power of the synchronous machine. Because the rotating exciter is a generator, this power could as well be made available with low, medium or high voltage. The voltage/current balance is chosen to best fit the available diodes characteristics. In principle the limitation in output current, due to the available diodes, can be overcome by setting two or more diodes in parallel. Actually this would result in a strong overload on diodes, and therefore the need for a heavy down-rating of them.
Nowadays, usually rotors operate at speeds of 3000 rotations per minute, leading to currents in the range of 2000 Ampere for energising the field winding of the synchronous machine. Usually the synchronous machine cannot be operated at higher rotational speeds due to a number of limitations such as instability of the shaft as well as high centrifugal forces on the rotating parts. In power generation, at a specified output, an increase of the rotary speed of a turbine however is associated with a decrease in size and costs. Efficiency, too, can be improved. Already, power generation turbines up to 70 MW are therefore connected to generators by way of gearing arrangements, so as to allow operation at higher rotary speeds.
It is well known that in particular at higher rotational speeds, the current induced in the rotor itself induces a reaction field, commonly called armature reaction. The actual magnetic field “seen” by the conductors of the rotor is consequently given by the superposition of the static magnetic field provided by the static outer core and this armature reaction. The superposition does not have the desired rectangular characteristic anymore but rather a distorted one, giving rise to unbalanced currents in the conductors of the rotor and correspondingly to ripples and peaks in the induced voltage pattern. These ripples and peaks can be critical in the sense that they lead to peak loads on the diodes used for rectifying the alternating current induced in the conductors of the rotor, which peak loads lead to an overload on the diodes and finally to a breaking of the diodes and to corresponding short-circuits.